vop_stdadvise(): restore correct handling of length == 0

[kib]

Summary

Switch to unsigned arithmetic to handle overflow not relying on -fwrap,
and specially treat the case of length == 0 from posix_fadvise() which
passes OFF_MAX as the end to VOP.  There, roundup() overflows and -fwrap
causes bend and endn become negative.  Clamp them at OFF_MAX/bsize
instead.

Also remove locals with single use, and move calculations from under bo
lock.

Reported by:    tmunro

[markj]

sys/kern/vfs_default.c

1122

[tmunro]

sys/kern/vfs_default.c

1119

I don't think we need this "Restore length" hunk. The round_page() call always just adds one to OFF_MAX (0x7fffffffffffffff -> 0x8000000000000000), so the change to uintmax_t is enough to make the existing endn = bend / bsize produce a correct positive answer.

[kib]

clamp is not needed